Dibasic 3,3'-di-nitro-4,4'-bipyrazole-1,1'-diides of K, Rb and Cs reveal metal-selective ring-π over ring-N coordination.

Three new alkali metal pyrazolate salts, namely, poly[[μ-3,3'-di-nitro-4,4'-bipyrazole-1,1'-diido]dipotassium], [K2(C6H2N6O4)] n , (1), its rubidium analogue [Rb2(C6H2N6O4)] n , (2), and poly[[[μ-3,3'-di-nitro-4,4'-bipyrazole-1,1'-diido]dicaesium] monohydrate], {[Cs2(C6H2N6O4)]·H2O} n , (3), suggest that the electron-depleted nitro-pyrazolates may be still functional as efficient π-donors toward late alkali metal ions. The three-dimensional structures of 1 and 2 are very similar, being governed by μ4-κ1:κ1:κ1:κ1 coordination of the pyrazolate anions as hard Lewis bases [K-N = 2.783 (3)-2.956 (3); Rb-N = 2.930 (4)-3.194 (4) Å]. In contrast, the structure of 3 reveals a dominant significance for the pyrazolate-π coordination, which produces two distinct motifs in the forms of finite Cs-π arrangement [Cs-ring centroid distance = 3.389 (3) Å] and one-dimensional -Cs-(π-Cs) n - sandwiches with slightly larger Cs-π separations at 3.474 (3) and 3.587 (3) Å. Weaker ion-dipole inter-actions with the nitro-O donors, at the distances approaching the sums of corresponding ionic radii, are relevant for the entire series and they complete typically high coordination numbers of the metal ions [CN = 8, 9]. These findings may find use in the construction of ion-selective receptors since the existing examples for the coordinative discrimination of the late alkali metal ions, beyond the ion-cavity size match considerations, are still rare.